1. Technical Field of the Invention
The present invention relates to an image processing apparatus and a medical imaging modality, both of which provide images of the internal structure of an object to be examined, and in particular, to the apparatus and modality capable of correcting beam hardening caused due to a contrast agent injected into the object.
2. Description of Related Art
As one conventional medical modality, there have been known modalities to obtain contrast-enhanced blood vessel images using a contrast agent. Such modalities include an X-ray imaging system directed to circulatory organs. The X-ray imaging system is equipped with, for example, an approximately C-shaped supporter (hereafter, referred to as a “C-shaped arm”), an X-ray tube, functioning as an X-ray generator, attached to one end of the C-shaped arm, an image intensifier, functioning as a detector, attached to the other end of the C-shaped arm, an image processor for processing acquired projection data, and other components. This system is also called an X-ray angiography system, and it enables X-ray imaging conducted during examination and operations, such as insertion of a catheter into an object, carried out by a doctor.
Another medical modality has been proposed as an IVR (Interventional Radiology)-CT system in which the foregoing X-ray angiography system and an X-ray CT scanner are incorporated in a combined manner. In this IVR-CT system, the CT gantry of the X-ray CT scanner and the C-shaped arm of the X-ray angiography system are operated in a mutually-related manner in the same diagnostic spacing. For instance, information about an object (which is for example a tomographic image or a blood vessel contrasted image) acquired by one of the systems is used to determine in a shorter period of time how to perform an X-ray examination in charge of the other system. Therefore, on inserting a catheter into a region to be examined of an object, it is possible to perform entire angiographic imaging, then to perform CT imaging to determine nutrient blood vessels to a tumor.
This IVR-CT system makes it possible to perform IVR. That is, an aneurysm, constriction and others are treated through surgery in a lower-degree invasiveness, under an X-ray fluoroscopic condition. In the IVR, a catheter is inserted to a portion to be examined. And if there is an aneurysm in the portion, a coil ejected from the tip of the catheter is placed within the aneurysm, while if a constriction is present in the examined portion, a balloon is loaded to be ejected and expanded in the constriction.
For conducting such operational techniques, it is extremely significant that blood vessels in a region being examined of an object are three-dimensionally grasped. The foregoing X-ray angiography system makes it possible to reconstruct a three-dimensional vessel structure from contrasted images obtained during operations. Hence 3D imaging based on the X-ray angiography system recently has become an indispensable tool for the IYR.
However, in such 3D imaging based on the angiography system that uses a contrast agent, there is a problem of artifacts caused in connection with blood vessels. In other words, when acquiring signals of a blood vessel running in a certain direction and reconstructing the image thereof, an angiographic image R1, which should be displayed originally, is deformed into a more compressed angiographic image R2, as shown in FIG. 1A. This compression problem becomes more noticeable when the foregoing “certain direction” agrees with a direction in which an X-ray path (X-ray transmittance length) becomes longer especially.
As to the 3D imaging based on the angiography system, there is another problem. To be specific, for imaging an aneurysm having a larger diameter as shown in FIG. 1B, reconstructed values at pixels residing within the aneurysm itself are likely to reduce. If such a phenomenon occurs, a certain inner area of an aneurysm is displayed without pixel values, that is, in a hollow state. Concurrently with this, other blood vessels that run around the aneurysm are displayed thinner than their true diameters.
It is considered that the primary cause comes from the fact that the X-ray provides X-ray multiple spectrums. That is, the cause is considered one type of beam hardening phenomenon, in which projection data based on a specific region of the X-ray spectrums that contributes largely to the absorption of a contrast agent decrease greater compared to projection data based on the remaining region of the X-ray spectrums. In general, concerning this beam hardening toward such elements as bones and soft tissues, a variety of types of correction techniques have been proposed. However, no correction techniques have been reported yet toward artifacts caused by the contrast agent.
An examination that uses a contrast agent is also carried out by the X-ray CT scanner and is called CTA (Computed tomographic angiography). The CTA uses a contrast agent which is injected from a vain. The density of such contrast agent is as small as approximately {fraction (1/10)}, compared to that of a contrast agent that is directly administered from an artery.
Incidentally, strictly speaking, the term “densities” used in this specification means a product of the density itself of an injected contrast agent and a thickness of an area (pixels) in which the contrast agent is present. Provided that the density of the injected contrast agent is constant, the “densities” correspond to a thickness of the area. It is therefore possible the “densities” can be interpreted as pixel values.
The X-ray CT scanner has therefore no problems about foregoing artifacts, because a lower-density contrast agent is used. In contrast, in the case of the X-ray angiography system, a catheter is inserted to a portion to be examined under a fluoroscopic condition in order to directly inject the contrast agent into the examined portion. Thus, a contrast agent is injected before its densities begin to decrease, so that the higher-density contrast agent often causes artifacts as described above, although such higher-density contrast agent is helpful in obtaining highly contrasted images.
Like the above, the IVR-CT system allows a doctor to inject a higher-density contrast agent from a catheter placed near to a portion to be examined, thus encountering the similar artifact problem as the above.